$$ \begin{pmatrix} \ddot{x}_1 \\ \ddot{x}_ 2\end{pmatrix} = \begin{pmatrix} \frac{k+K}{m} & \frac{-K}{m} \\ \frac{-K}{m} & \frac{k+K}{m} \end{pmatrix} \begin{pmatrix} x_1 \\ x_2 \end{pmatrix}$$